NRAC Publication No. 201-2010

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Softshell Culture: Basic Biology and General Culture Considerations Scott Weston and Joseph K. Buttner, Northeastern Center and Department of Biology, Salem State University, Salem MA Brian F. Beal, University of at Machias and Downeast Institute for Applied Marine Research and Education, Beals, ME

The softshell clam, Mya arenaria, (Figure 1) is an extends through the sediments to the sedi- ecologically important bivalve mollusk that is harvested ment/water interface. Cilia associated with the recreationally and commercially from coastal waters of move oxygenated water with microscopic , such as the western North Atlantic. As with all bivalve mollusks, , and small organic particles, they possess two valves or shells that are connected by a hinge. The pair of whitish/gray shells surrounds and pro- tects a , visceral mass and muscular foot. The elongate shells are relatively thin and brittle, leading to the name “softshell.” Unlike hard and , the softshell clam cannot close its shells completely, result- ing in a gape most notable at the anterior and posterior ends. As such, softshell clams are damaged more readi- ly and can have a shorter shelf-life than do similar-size oysters or hard clams. A shelf-life of 7 to 10 days can be expected when the animal is kept cool, approximately 3 to 4oC, and moist. The softshell clam may exceed 10 cm in shell length (SL) and lives burrowed in sediments. The clam moves by using its relatively small, muscular foot. Clams less than 25 mm SL use the muscular foot to burrow and to move laterally on the sediment surface; larger clams, which tend to be sedentary, use the muscular foot to move vertically in the sediments. Softshell clams pos- sess a single long, fleshy siphon composed of a fused, Figure 1. Softshell clams are typically white or gray and larger incurrent and a smaller excurrent tube. The fused cannot close completely. (Photo: Joe Buttner) through the incurrent tube into the visceral mass. The gills Life Cycle sieve out or filter food and other particles, and send it to the labial palps. The palps sort food items, directing some Softshell clams in nature routinely live for 10 to 12 to the mouth for ingestion. Other food items and non-food years and have been documented to live for 28 years. items are bundled in expelled through the excur- Sexes are typically separate, but a small percentage of a rent tube as . Metabolic wastes and are population may be hermaphroditic. Male and female also expelled via the excurrent tube. Large clams may fil- clams are identical externally. The only non-intrusive ter 50 L of water per day depending on temper- way to determine the sex of an individual is to observe atures. When disturbed, the siphon can be retracted its gametes released at spawning. Clams can develop quickly and a squirt of water is emitted. gametes at ~20 mm SL; by 30 mm SL most are able to reproduce. As water warms in the spring, gametes start Distribution to ripen or mature. This process can take 8 or more weeks. Spawning generally occurs after inshore water temperature reaches at least 10ºC. A second may The softshell clam naturally ranges in coastal waters occur in the late summer or early fall in more southerly of the western North Atlantic from Labrador and the waters of the western North Atlantic, as water tempera- Canadian Maritimes south along the Atlantic shore to ture drops below 25ºC. One female can produce and North Carolina, but is not common in waters south of release in excess of one million per spawn. Eggs Maryland. Outside its natural range, the clam is often con- are round and typically 40 to 50 µm (0.04 to 0.05 mm) sidered a nuisance . Softshell clams were first in diameter. are 2 to 3 µm in length. observed on the U.S. Pacific coast in San Francisco in 1874. They have subsequently spread northward into Ore- Fertilization occurs externally as eggs and sperm are gon, Washington, British Columbia and waters. In released into the water column and encounter each other. Europe, this species has become established from Nor- After fertilization, the eggs pass through three - way to Portugal and has been introduced into the Mediter- ic larval stages: the short-duration trochophore, the ranean with reports of its existence along the northern and finally the pediveliger (Figure 2). Tro- Adriatic, western Sicily, and the Saronikos Gulf in chophore larvae are characterized by the presence of rel- Greece. atively large apical cilia. A mouth, gut, and anus are present at this developmental stage, but no valves or shells occur. Veliger larvae are characterized by the pres- ence of a velum and very thin valves. The ciliated velum enables larval clams to swim and capture food. The pedi- To protect their thin and brittle shells, softshell clams veliger is the last larval stage before the animal settles to are normally found buried in a wide variety of sediment the bottom. It can be quite variable in duration as the types in protected bays and . The clam grows best young clam may metamorphose quickly or remain in substrates of mixed and silt. It will colonize and planktonic for several weeks depending on various envi- can attain high densities in clay, mud, silt and gravel sed- ronmental and biological cues. Temperature, salinity and iments. Clams in coarse sediments, such as gravel, typi- food availability affect duration and success of the larval cally have thicker and more deformed shells compared to stage. Typically, the larval stage is 8 to 21 days in dura- clams in finer sediments. Densities can exceed 100 mar- tion. The stage ends as planktonic larvae metamorphose 2 ket-size individuals (≥ 51 mm SL)/m . into benthic, juvenile clams, which lack a velum and possess a single-chambered siphon as well as a function- The eurythermal and euryhaline softshell clam al muscular foot as the means of locomotion. tolerates temperatures from -2 to 28ºC and salinities Recently metamorphosed benthic clams are small with a between 4 to 33 ppt. They prefer cooler temperatures, shell length of ~200 µm (0.2 mm). with an optimum temperature of 17 to 23ºC. North of Cape , Massachusetts the clam is predominantly Both the pediveliger and recently-metamorphosed intertidal in distribution, burrowing in the sediments to clams produce elastic byssal threads in a gland within a depth of 30 cm or more. South of the clam the foot. Byssal threads attach the clams to small objects becomes increasingly subtidal and burrows deep in the such as sand grains in the substrate. Young clams can substratum to avoid elevated temperatures. They can be also detach from their byssal threads if necessary and found in waters ≥ 200 m deep. can move along the surface of the sediment pushed by tidal or wind-driven currents. Softshell clams lose their

2 Figure 2. Life cycle of softshell clam. (Drawing by Virge Kask, UConn Biological Illustration.) ability to produce byssal threads as they approach 20 Predation and Disease mm SL. Juvenile clams up to ~20 mm SL are lightweight and do not bury deeply, which means they may be dis- Softshell clam populations are adversely affected by rupted and transported by waves or currents, despite predation, environmental stress and disease. Predators being attached to sand grains by byssal threads. Some- vary with life stage. Larval clams are preyed upon by times, clams appear at the sediment surface to avoid plankton and , which if abundant can decimate a predators, anoxic conditions, or for other reasons. If this cohort. Post metamorphic clams are ingested by occurs on sunny days during periods when the is nemertean worms, segmented worms, fish, aquatic birds, low, the that is exposed to the sun may dry. When , horseshoe crabs, drills, moon and the tide returns, the clam will float because of trapped air meiofauna. Surviving clams minimize predation by or increased water tension associated with drying out. growing large and by burrowing deeper into the sedi- The clam may be transported toward the upper intertidal ments. Elevated temperatures during hot summers have area. This action tends to help clams establish them- been associated with mortalities in intertidal populations selves near the upper of some flats. and even subtidally in . Few diseases Softshell clams grow relatively fast. Under optimal have been identified that directly affect softshell clams, conditions they can grow up to 8 mm in a month during unlike oysters and hard clams. Neoplasia, a leukemia- their first summer. Growth is mainly determined by tem- like disease, is relatively common but occurs at very low perature, food supply, current, density and sediment incidence levels (typically less than 5%). At times, how- type. Most growth occurs during the spring, summer ever, neoplasia has been known to cause massive mortal- and fall, with little to no growth in the winter, especially ities. Populations in , and in Maine and eastern Canada. Some clams in more in Chesapeake Bay have experienced catastrophic die- southerly waters may become sexually mature by the offs due to neoplasia. spp., the causative agent end of their first summer, and the vast majority of soft of Dermo, has been identified in softshell clams, but its shell clams are ready to spawn the year after they settle impact on clam health remains unclear. into the sediment.

3 Few diseases have been attributed to inges- tion of softshell clams. The clam may serve as a vector for bacteria or paralytic poisoning. Federal and state regulations reduce the likelihood of contact- ing disease if clams, or other commercial shellfish, are purchased from a reputable source.

Harvest

Throughout its western Atlantic range from the Que- bec North Shore and Prince Edward Island to the Chesa- peake Bay, softshell clams have historically been harvested commercially and recreationally. Pollution and inconsistent sets have restricted harvest. Outside its natural range, softshell clams supported modest com- mercial activities in the past (e.g. California, from 1880s to 1948; Oregon, 30 tons in 1980; Washington, 180 tons Figure 4. Hydraulic rigs are used to harvest softshell in 1985). Today, clams outside their natural range are clams from subtidal waters. (Photo: Dale Leavitt) harvested recreationally for food and bait, but little or no commercial harvest occurs. Clams are typically harvest- in their shells, piled on a plate and ingested whole by the ed from intertidal waters by hand forks, rakes, or hoes consumer, after removing the shells and tissue that sur- (Figure 3). Rakes, plungers, forks and hydraulic rigs, rounds the siphon. Commonly, clams are dipped in where legal (Figure 4), are used to harvest clams in sub- melted or other sauces before being consumed. tidal sediments. According to the National Marine Fish- are shucked, breaded and deep-fried whole. eries Service (2009), approximately 3.8 million pounds Preferred size for steamed or fried clams is 63 to 76 mm were harvested in 2008 (worth in excess of $21 million). SL. The edible portion of softshell clams (35% by weight) greatly exceeds that of other commonly con- Marketing sumed bivalve mollusks such as oysters (13%), (18%), and hard clams (18%). Softshell clams are high Softshell clams are marketed and consumed almost in minerals, such as , , magnesium, potassium, exclusively as a cooked product, either steamed or copper, and phosphorus. These minerals help build breaded and fried. are boiled or steamed healthy cells, promote immune reactions, make proteins, promote muscular strength, healthy skin, and help build healthy bones. Twenty steamed clams with- out melted butter have 133 calories with 0.2 g of saturat- ed , 23 g of and 60 mg of cholesterol. Aquaculture

Little interest in aquaculture of softshell clams exists south of , but several private commer- cial and many public aquaculture initiatives exist in , particularly Maine and Massachusetts, and in Canada, particularly Quebec and New Brunswick. - lic aquaculture targets restoration and enhancement of selected flats harvested by commercial shellfishers. Pri- vate aquaculture is typically pursued on modest-size leases, two acres or so in size. Figure 3. Softshell clams are harvested from intertidal Public and private aquaculturists either purchase and waters by hand rakes and stored in bags for transport to stock hatchery-produced juvenile clams or entice setting wholesalers. (Photo: Joe Buttner) of small naturally produced clams under propylene

4 netting commonly installed to prevent predation by material, passing it either to the mouth for digestion or crabs and birds. Small clams also can be collected from expelling it via the excurrent siphon. one location and transferred to another location. Growth rates can vary significantly even within a bed of clams. Mantle – One of three structures found in and used to Time required to attain market size depends upon size of characterize mollusks. The mantle is a thin cloak of tis- clam stocked, time stocked (before, during or after sum- sue that surrounds the visceral mass and secretes the shell, if one is present. It is also involved in respiration, mer) and location. Typically, 1.5 to 2.5 years are feeding and reproduction. required for 10 to 15 mm SL clams stocked in southern New England waters during late spring/early summer to Meiofauna – Small that live in and on the sedi- attain market size. In Canada, where low temperatures ments that typically range in size between 0.064 mm and and short growing seasons limit growth, the time 0.5 mm. required to attain maturity and market-size is increased to 4 to 6 years. Details on spawning and culturing soft- Metamorphosis – A complete or marked change in the shell clams are described in NRAC publications 202- form of an animal as it develops into an adult. In clam 2010 Softshell clam Culture: Hatchery Phase, culture, metamorphosis refers to the period in which the Broodstock Care through Seed Production and 203-2010 veliger larvae gains a foot, becoming a pediveliger, then Softshell Clam Culture: Nursery and Growout Methods. loses its swimming organ called the velum, and settles to become a miniature benthic clam.

Glossary Muscular foot – One of three structures found in and used to characterize mollusks. Composed primarily of Anoxic – Relating to or characterized by a severe defi- muscle tissue, the muscular foot is used for locomotion. ciency of oxygen. Softshell clams use the foot to move in and on sedi- ments. Anterior – Pertaining to or toward the mouth end of the body. In a softshell clam the mouth is located above the Pediveligers – Larval mollusks that have developed a foot in the mantle cavity, at the opposite end of the small foot. The foot can be extended beyond the shell siphon. and used to travel along and probe the substrate.

Aquaculture – The farming of aquatic organisms such Planktonic – Organisms, including algae and clam lar- as fish, , mollusks and aquatic plants. vae, that float or drift in fresh or water. While some forms of plankton are capable of independent movement Byssal threads – Sticky threads or filaments of protein and can swim up to several hundreds of meters vertical- produced by and clams used to attach them- ly in a single day, their horizontal position is primarily selves to substrates. determined by currents in the body of water that they inhabit. Cohort – A group of individuals in a larger population that are of similar age or size. Posterior – Pertaining to or toward the anus end of the body. In a softshell clam, the anus connects to the excur- Euryhaline – References a mollusk or other organism rent tube of the siphon. Soft shell clams live in the sedi- that tolerates a wide range of salinities. ments with their posterior end nearest the sediment/water interface. Eurythermal – References a mollusk or other organism that tolerates a wide range of temperatures. Pseudofeces – The filtered, undigested material that is discharged because it is not food or the clam is full. The Gametes – Reproductive cells that unite during sexual rejected particles are wrapped in mucus and expelled out reproduction to form a new cell called a zygote. In of the siphon. In a hatchery it can be a sign of overfeed- clams, male gametes are sperm and female gametes are ing clams. eggs (or ova). Shell length (SL) – Measurement of the longest shell Hermaphroditic – Able to produce both male and dimension in bivalve mollusks. In softshell clams shell female gametes. length measures the distance from the anterior to poste- rior ends of the shell. Labial palps – Paired ciliated triangular flaps on either side of the mouth in bivalves. They sort ingested

5 Spawning – The release of eggs or sperm into the water References column for the purpose of reproduction. Anonymous. Undated. Guide to the Exotic Species of Trochophores – The larval stage in bivalves that devel- San Francisco Bay, Mya arenaria Linnaeus, 1758. ops 12 to 20 hours after fertilization and lasts for 12 to http://www.exoticsguide.org/species_pages/m_aren 24 hours. They are 50 to 60 µm pear-shaped spheres sur- aria.html. rounded by a girdle of cilia (fine hairs) with an apical tuft of cilia that is used to swim. Trochophores are Beal, B.F. 2005. Soft-shell clam, Mya arenaria, mari- equipped with a mouth, stomach and anus. This is the culture in Maine, USA: opportunities and chal- developmental phase that precedes the veliger. lenges. Bulletin of the Aquaculture Association of Canada. Special Publication No. 9:41-44. Velum – A structure found in planktonic, larval mol- Calderon, I. (ed). 2007. Atelier de Travail sur l’Elevage lusks. It is a swimming organ that can be retracted into de la Mye Commune, Institut des Science de la Mer the larval clam’s transparent shell. The ciliated velum also enables larval clams to capture food. de Rimouski, 20 au 22 Avril 2005. Les Publications de la Direction de l’Innovation et des Technologies, – Larval, planktonic mollusks with shells and a Compte Rendu No. 32, Ministere de l’Agriculture, swimming organ called a velum, which is replaced by des Pecheries et de l’Alimentation du Quebec, the siphon(s) once they become benthic organisms. This Gaspe. is the developmental phase that follows the trochophore. Cape Cod Cooperative Extension. 2004. A Report upon the Soft-shell Clam of Massachusetts, in The Visceral mass – One of three structures found in mol- Works of David L. Belding M.D. Biologist. Early lusks and used to characterize them. The visceral mass 20th Century Shellfish Research in Massachusetts, contains many important organs and systems including digestive, excretory and circulatory. Barnstable, MA. Newell, C.R. and H. Hidu. 1986. Species Profiles: Life Histories and Environmental Requirements of Acknowledgments Coastal Fish and (North Atlantic), Softshell Clam. Biological Report 82 (11.53), TR This publication was prepared EL-82-4. with partial funding from the Pariseau, J., B. Myrand, G. Desrosiers, L, Chevarie and Northeastern Regional Aquacul- M. Giguere. 2007. Influences of Physical and Bio- ture Center (NRAC) as part of logical Variables on Softshell Clam (Mya arenaria projects USDA NRAC 2006-385- Linneaus 1758) Burial. Journal of Shellfish Biology 17065 and USDA NRAC 2007- 26(2): 391-400. 385-18589 from the Department of Agriculture (USDA), National Institute of Food and Agriculture (NIFA) and from the Massachusetts Depart- ment of Agricultural Resources (MDAR). The authors gratefully acknowledge NRAC’s, USDA, NIFA’s and MDAR support. Any opinions, findings, conclusion or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of U.S. Department of Agriculture, Salem State University, Northeastern Mas- sachusetts Aquaculture Center, the Massachusetts Department of Agricultural Resources and the Universi- ty of Maryland. This publication was prepared with the assistance of Department of Biology and Salem State University. The work of four anonymous reviewers who helped to strengthen this document is much appreciated.

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